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Ian
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Topic 194508

plains of reality and other things to think about.

I know this is of topic, but what are the chances of the moon not revolving subject to our Planet?

Ian

Mike Hewson
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orbits - moved

Quote:

plains of reality and other things to think about.

I know this is of topic, but what are the chances of the moon not revolving subject to our Planet?

Ian


On the assumption of our current understanding of gravity, the Earth-Moon system ( and the Solar system generally ) is stable and will be so for many eons hence. While there may be doubt about exactly where in an orbit a body will be in the future, there's no doubt it will be in an orbit much like the present one. There is a 'sensitive dependence upon initial conditions' for such statements however - meaning that a small error in any current measurement ( as an input to a predictive model ) will lead to substantial uncertainties in relatively short time spans. It would take some external body of appreciable size to rattle the current status quo. Has anyone read about 'Nemesis', a hypothetical brown dwarf star that may be lurking in our neighbourhood? Well, you can't rule it out .... :-)

There will be, and has been, a very very gradual increase in the Earth moon distance due to tidal effects. Essentially the sloshing of the oceans against the continents acts like a brake shoe upon the Earth and gradually slowing it's rotation ( so the days will have more seconds in them ). Angular momentum is conserved by 'pushing' the moon to a higher orbit.

The pattern of the moon's behaviour has caused much angst for theoreticians over the centuries. Newton himself despaired of the problem - and he only had three bodies in his model ( Earth, Moon and Sun ). Poincare had a crack at it too. At one time it was a leading contender for solving the longitude problem for sailors, but was sufficiently unpredictable in that role. There is an outstanding book, which I've delightfully read many times, called 'Newton's Clock: Chaos in the Solar System' by Ivars Peterson that discusses this entire area.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Bikeman (Heinz-Bernd Eggenstein)
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Hi Mike, I wonder if the

Hi Mike,

I wonder if the question of the OP might have been related to the fact that the moon doesn't rotate *as seen from Earth*, as we (more or less) always see the same side of the moon. This is called synchronous rotation and is not something unique to the Moon-Earth system, it's rather pretty common and not a strange coincidence.

CU
Bikeman

Andrew Lorimer
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The moon pleas to us. Pale

The moon pleas to us.

Pale yellow it yearns. Thus we do to.

We want things to be alright.

Mike Hewson
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RE: Hi Mike, I wonder if

Message 94609 in response to message 94607

Quote:

Hi Mike,

I wonder if the question of the OP might have been related to the fact that the moon doesn't rotate *as seen from Earth*, as we (more or less) always see the same side of the moon. This is called synchronous rotation and is not something unique to the Moon-Earth system, it's rather pretty common and not a strange coincidence.


There's a thought! Yup, synchronous locking is almost inevitable in a system like ours. When it's described as due to tidal effects it's not referring to Earth's oceans however.

'Tidal' as a general term is for effects that occur if there is a gravity gradient of significance - as there nearly always is. A gravity gradient means that some nearby positions in space differ in the force of gravity due to some other body. So if I'm on the nearside of the moon I will be attracted to the Earth a bit more than if I'm on the farside. The substance of the moon while 'solid' does have a bit of give and that difference in forces around it's girth leads to an egg-like shape and not a sphere. In turn the rotation of the moon in that shape affects the rate of rotation. Eventually the moon's rate of rotation about it's own axis becomes equal to the rate of revolution of it's orbit around Earth - so a given face will be presented to any Earth observers. It's hard to give an easy analogy for that, it just is that way.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Rod
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RE: RE: Hi Mike, I

Message 94610 in response to message 94609

Quote:
Quote:

Hi Mike,

I wonder if the question of the OP might have been related to the fact that the moon doesn't rotate *as seen from Earth*, as we (more or less) always see the same side of the moon. This is called synchronous rotation and is not something unique to the Moon-Earth system, it's rather pretty common and not a strange coincidence.


There's a thought! Yup, synchronous locking is almost inevitable in a system like ours. When it's described as due to tidal effects it's not referring to Earth's oceans however.

'Tidal' as a general term is for effects that occur if there is a gravity gradient of significance - as there nearly always is. A gravity gradient means that some nearby positions in space differ in the force of gravity due to some other body. So if I'm on the nearside of the moon I will be attracted to the Earth a bit more than if I'm on the farside. The substance of the moon while 'solid' does have a bit of give and that difference in forces around it's girth leads to an egg-like shape and not a sphere. In turn the rotation of the moon in that shape affects the rate of rotation. Eventually the moon's rate of rotation about it's own axis becomes equal to the rate of revolution of it's orbit around Earth - so a given face will be presented to any Earth observers. It's hard to give an easy analogy for that, it just is that way.

Cheers, Mike.

If I understand.. If you cut the moon in half... The half that is facing the earth has more mass than the far side half due to gravitational pull of the earth and is mostly responsible for the present synchronous locking effect of the earth moon system

Hmm: I wonder if the same thing happens with close orbiting binary neutron stars

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Mike Hewson
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RE: If I understand.. If

Message 94611 in response to message 94610

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If I understand.. If you cut the moon in half... The half that is facing the earth has more mass than the far side half due to gravitational pull of the earth and is mostly responsible for the present synchronous locking effect of the earth moon system


Sorry, perhaps 'egg' was the wrong word. Chicken eggs have 'blunt' and 'sharp' ends. I really meant prolate spheroid! An Aussie/American football has that shape. But it's a dynamic thing as there are rotations. Or put another way there is inertia involved. So in effect there is a symmetric elongation of the moon from a sphere to, in the extreme, something like a cigar.

I'll try and describe the situation in the frame of reference of the moon ( which is not inertial ), as opposed to a distant observer ( which would be inertial ). The moon is held at a given radius from Earth due to two effects - gravity pulling towards Earth and 'centrifugal' force which is in the opposite direction. [ As I'm moving with the moon I can use that 'centrifugal' term to indicate what is seen/labelled as inertia in the distant frame ]. In this manner of description the gravity force is more if you get closer to the Earth, while the centrifugal component is more the further you are away from the Earth. Recall the feeling on a roundabout. The centre of the moon will have those effects equal and opposite. But now a prolate moon is going to have a nearside 'pointy' end more attracted to the Earth than the centrifugal force throws it outward, whereas the farside 'pointy' end is less attracted to the Earth with centrifugal force being stronger. So the nett effect for the nearside is towards the Earth, and for the farside away from the Earth. A bit of vectors reveals that this causes the moon to turn around it's own axis until both the pointy ends, and the moon's centre, are along a direct line outwards from the Earth's centre.

It's actually more horrible than that, because of delays ( ie. relativity trumping Newton ) and that the moon has variations in density - so those lumpy bits alter the picture. They discovered a fair bit of this with the moon landings, requiring alterations to their orbits to compensate. What might have been circular orbits of the craft around the moon at about 60 miles above the surface became more elliptical ( or worse, irregular ) as the extra mass under the surface attracted the capsules more when they passed by, making them go lower and faster. Compared with the less dense areas which attracted less and hence the craft went higher and slower. This is oh so important if you want to arrive at a certain height, with a certain velocity, in order to begin some planned descent to a specific point on the surface. And of course when you come back up again to re-engage the lunar ascent stage with the command module. They discovered these 'mass-cons' or mass concentrations, when the missions began arriving early & low or late & high at particular surface landmarks. Later satellites I think have mapped the density variation pretty completely.

Quote:
[/Hmm: I wonder if the same thing happens with close orbiting binary neutron stars


Certainly could. In the Earth/Moon case the Earth is rather more massive. So the tidal effect on the Moon due to the Earth is very much greater than the tidal effect on the Earth due to the Moon. Thus if you have bodies of more or less similiar mass the above effects are quite symmetric for the two bodies. So if I'm on Neutron Star A, looking directly overhead up at you on Neutron Star B ( we're both on our respective nearsides ) - and you're doing the obverse - then we'd better both get used to it, as it won't change for a while at least! :-)

If you take a black hole and a star passing nearby, it won't necessarily be captured by it. But if it passes within about 10 Schwarzschild radii the star will be utterly disrupted by tidal effects. ( the Schwarzschild radius is where the event horizon is, effectively the edge of the hole ). What was a spherical star becomes a huge smear of gas across the neighborhood and some of the gas on the nearside is gobbled up. There's no chemical bonding in stars to give their shape ( unlike rocks and stuff ) so it's easier to rip up. The ultimate bug splat!! :-)

Cheers, Mike.

( edit ) Notwithstanding that me on any neutron star is quite a flat proposition!! :-)

( edit ) One could describe the tidal scenario in inertial terms from a far viewpoint. You'd get the same result though. Instead of mentioning centrifugal force and an 'equilibrium' state, you'd have only gravity acting towards Earth but we haven't equilibrium - there's a nett acceleration toward Earth. In which case the nearside is being accelerated slightly more than the farside, but the farside has a greater tangential velocity! So for the farside, a lesser acceleration on a faster component will be deflected less in a given time. For the nearside you have a greater acceleration with a slower component, and thus a greater deflection in a given time. So the overall effect is as above with the nearside moving closer to the Earth, and the farside aligned away.

( edit ) I don't think the mass-cons altered whether a given landing would/not occur, but certainly altered the trajectory, fuel burns, margins and whatnot.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

ML1
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RE: RE: If I understand..

Message 94612 in response to message 94611

Quote:
Quote:
If I understand.. If you cut the moon in half... The half that is facing the earth has more mass than the far side half due to gravitational pull of the earth and is mostly responsible for the present synchronous locking effect of the earth moon system

Sorry, perhaps 'egg' was the wrong word. Chicken eggs have 'blunt' and 'sharp' ends. I really meant prolate spheroid!...


My simplistic view is that as the moon used to rotate faster than it's orbit around the earth, tidal forces squished the moon and so caused heating and drove other energy sapping effects in it's structure. That heating and other effects dragged the energy out of the moon's rotation and so eventually it slowed to a halt.

Energy is neither created nor destroyed... It comes from somewhere and it goes somewhere all in equal measure.

Cheers,
Martin

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Mike Hewson
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RE: My simplistic view is

Message 94613 in response to message 94612

Quote:

My simplistic view is that as the moon used to rotate faster than it's orbit around the earth, tidal forces squished the moon and so caused heating and drove other energy sapping effects in it's structure. That heating and other effects dragged the energy out of the moon's rotation and so eventually it slowed to a halt.

Energy is neither created nor destroyed... It comes from somewhere and it goes somewhere all in equal measure.


You're quite right, that is also an effect. There are many factors, on varying timescales and of varying magnitude. It's a fascinating topic and the exact answer depends on the scenario. The internal movements of one part of the moon with respect to another is friction, thus heat. So the shape is not static, it's not really a 'rigid' body but only approximately so. So if the shape changes all of the effects mentioned so far act on in an evolving manner.

Jupiter has a satellite Io, that orbits close enough in to the fairly deep gravity well so there is a pretty significant difference from one side to the other. It is continually kneaded like a lump of dough, the core heats and that energy comes to the surface as eruptions. So Io is always pfoofing off stuff. There's a complex interaction with Jupiter's magnetic field too.

Binary stars systems of all sorts have a particular behaviour which many pass through at some time. One of the pair is quite dense and smaller, the other bigger. There are points between the two where, in an inertial sense ( centrifugal again ), the force from one balances the force from the other. So gas and whatnot will tend to go to one body or the other depending on which side of these points it lies. The sum of all these points defines a globular shape in space called a Roche lobe. It's a hypothetical surface, somewhat like a line drawn on a map. At the nearside to the smaller partner there is a kind of 'funnel' region of the bigger body's Roche lobe. The big star bleeds gas to the smaller one through that region. All manner of things happen to the gas infalling upon the condensed star, but eventually the larger star is sapped of so much mass the channelling stops. And then this much diminished star evolves some more ....... and so on.

Another point of interest is that while energy gets converted to different forms or flavours, some of them apparently subtle, momentum has only one 'form'. So momentum always 'looks like' momentum, and it can't be hidden. Similiarly for angular momentum.

Cheers, Mike.

( edit ) What I mean is that momentum is evident at any scale, but some energies are only obvious at certain scales.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Martin Ryba
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RE: Hmm: I wonder if the

Message 94614 in response to message 94610

Quote:
Hmm: I wonder if the same thing happens with close orbiting binary neutron stars

Not strongly enough to be of note until they get really close. Tides are proportional to the sizes of the objects over their distance apart. Neutron stars are only the size of a city.

Also, the dissipation caused by tides strongly depends on the equation of state. With neutron stars, we know only a little. For instance, rock/magma has a lot of viscosity; solar material has less (the vicosity is generated by magnetic fields). Neutron stars...who knows? Nuclear material has little viscosity (the "liquid drop" model seems to work well) but the magnetic fields are strong so that will do something.

Maybe a LIGO-associated theorist has worked some of this out...for instance, if tides caused a bulge that took some time to damp out during the rotation like how the oceans slosh around from tides, that would cause an asymmetry that would "ring" a bit if the EOS is bouncy. That would generate gravitational radiation at various harmonics of the spin frequency along with harmonics of the "ringing" frequency. That's what E@H would potentially hear in the data we're crunching. Of course, it may take a close binary to cause enough tidal warping, which would smear the tone signals and therefore would require a binary searching algorithm like that used with the ABP1 data. The problem is the data span of LIGO data needed to get enough SNR is many orbits, which makes the close binary search impractical.

(Oh, oh, rambling by theorizing while typing)

"Better is the enemy of the good." - Voltaire (should be memorized by every requirements lead)

Martin Ryba
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RE: Another point of

Message 94615 in response to message 94613

Quote:
Another point of interest is that while energy gets converted to different forms or flavours, some of them apparently subtle, momentum has only one 'form'. So momentum always 'looks like' momentum, and it can't be hidden. Similiarly for angular momentum.

Yes, angular momentum is conserved. Thus, the lunar tides are slowly slowing down the earth's rotation (why we now have to insert leap seconds into UTC every once in a while...the atomic standard second was defined in the 1960s). That angular momentum is transferred to the earth-moon orbit, which means the moon is slowly getting farther away (measurable thanks to laser ranging measurements off the mirrors placed on the moon by the Apollo astronauts).

"Better is the enemy of the good." - Voltaire (should be memorized by every requirements lead)

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